It often becomes desirable when producing an oil well, to treat the well to enhance and increase flow, such as by applying an acidic solution to the producing formation under pressure or by applying a hydraulic solution to the formation under extremely high pressure in order to fracture the formation. In the past, it was necessary to "kill" the well by pumping a fluid, typically mud or water, into the well until sufficient hydrostatic pressure was obtained to overcome the pressure of the formation and prevent the blowing out of fluids from the well. The wellhead was then removed, and the necessary treating apparatus tied into the production tubing. After treatment, the well then had to be swabbed to re-institute production. This cumbersome process is superceded by a wellhead isolation system disclosed in U.S. Pat. No. 3,830,304 entitled "Wellhead Isolation Tool and Method of Use Thereof," issued to Alonzo E. Cummins and assigned to Halliburton Company, Duncan, Okla. The apparatus described therein provides means for directly communicating with production tubing without the removal of the wellhead, killing the well, or swabbing the well after treatment. This is accomplished by providing a hollow high pressure mandrel slidably engaged within a high pressure casing, the casing being adapted for sealing contact with the wellhead and the mandrel being adapted for selective sealing engagement with the upper end portion of the production tubing below the wellhead. The mandrel can be extended or retracted for engagement or disengagement with the production tubing without necessitating the removal of the wellhead. The treating fluids can then be supplied to the well through the mandrel directly into the production tubing of the well without subjecting the wellhead to the high pressures in the mandrel and production tubing. When the mandrel is extended in order to supply fluids to the well, the end of the mandrel is inserted within the production tubing.
As fluids, particularly sand-laden fracturing fluids, are pumped into the well through the mandrel of the wellhead isolation tool, serious erosion results at the exit end of the mandrel and on the interior of the production tubing adjacent the end of the mandrel. This erosion is due to a combination of a number of different factors. First, one is dealing with high flow rates utilizing particulate-laden abrasive fluids. Combined with this factor is the creation of turbulence when the fluid exiting the mandrel encounters the necessarily larger inside diameter of the production tubing, and abruptly loses velocity. The tendency of flowing fluids to expand or flare outwardly at and following the point at which lateral constraints on it are removed, such as at the exit end of the diffuser is another contributing factor to erosion. This tendency is further compounded in gel fluids, which have a tendency to swell and flare to an even greater degree, a phenomenon known as "die-swell." Several prior attempts have apparently been made to reduce the effects of the aforesaid phenomena, in the hope of reducing tool and tubing erosion.
U.S. Pat. No. 4,023,814 issued to Charles A. Pitts and assigned to the Dow Chemical Company, discloses a "packer cup assembly," which is coaxially attached to the lower end of the mandrel of a wellhead isolation tool. The exit end of the bore of the packer cup assembly diverges outwardly at an angle of approximately 10.degree. from the axis of the tool. No specific reference is made to the reduction of erosion, if any, that this exit angle produces.
U.S. Pat. No. 4,111,261, issued to Owen Norman Oliver and assigned to Halliburton Company, discloses a "guide nose" at the end of a wellhead isolation tool mandrel, the bore of which diverges from the axis of the tool at a point near its exit end at a gradual and uniform angle of not more than 3.degree. and preferably 2.degree. from the axis of the tool. This divergence is intended to provide a gradual reduction in velocity of fluid flow as the treating fluid approaches the exit end of the guide nose and reaches the production tubing.
The prior art, while possibly reducing tool and tubing erosion, still possesses certain disadvantages, in that erosion at the exit end of the packer cup assembly or guide nose has not been eliminated or reduced to an extent whereby the apparatus can be reliably used on a large volume job, or for more than one treatment without replacement of the packer cup assembly or guide nose. Furthermore, erosion of the production tubing adjacent the exit point of the fluid from the wellhead isolation tool has not been consistently reduced to a level whereby the operator is assured, particularly when multiple treatments are to be employed, that the tubing is not dangerously eroded. This problem is particularly serious in view of the fact that there is no practical way to inspect the production tubing and determine the possible extent of erosion from each treatment; a failure in the production tubing exposes the wellhead and valve assembly, usually rated at no more than 10,000 PSI, to pressures often in excess of 20,000 PSI without prior warning and with disasterous consequences. Even if the wellhead withstands the sudden pressure increase, the severed string of production tubing may drop down the well. With increased emphasis on well treatment due to the gradual but steady depletion of production from existing wells, the ability for the operator to assure himself that treatment can be effected safely and expeditiously, an assurance which the prior art has not been able to give, becomes of paramount importance.
In contrast, the present invention overcomes the disadvantages and limitations of the prior art by providing a guide nose, preferably referred to as a "diffuser," which markedly reduces both tool and tubing erosion to a minimum. The present invention contemplates a diffuser for attachment to the lower end of a wellhead isolation tool mandrel, the diffuser having a bore coaxial with that of the mandrel. The diameter of the upper portion of the bore of the diffuser is equal to that of the mandrel, and the diameter of the lower portion at its point of exit from the diffuser is only slightly less than the interior diameter of the upper end portion of the oil well production tubing. The bore between the upper portion and the lower portion at its exit point initially diverges at a gradual and uniform rate to a point prior to the exit end, after which it may maintain a constant diameter, converge at a gradual and uniform rate, or initially maintain a constant diameter for an axial distance and than converge at a gradual and uniform rate.
The initially diverging portion of the diffuser bore flares outwardly in a downward (as the tool is installed on the wellhead) direction from the axis of the diffuser for a distance along the bore. At that point, in one embodiment, the bore may then maintain a constant diameter to its lower, or exit, end. Alternatively, the bore may initially diverge as previously stated, and then converge at a "negative angle" to the axis of the diffuser, this convergence continuing to the exit end. In a third embodiment, the bore may diverge for a distance, then remain constant for a second distance, finally converging to the exit end.
The diffuser possesses seal means on its exterior for fluid sealing with the inside of the production tubing, such seal means being more fully described in copending U.S. patent application Ser. No. 065,654 filed on Aug. 10, 1979 and entitled "Seal System for Wellhead Isolation Tool Diffuser" by Thomas J. Luers and Richard L. Giroux. This fluid seal of course, isolates the high pressure in the production tubing from the relatively low-strength wellhead and valve assembly.
The aforementioned combinations of diffuser bore angles significantly reduce tool and tubing erosion by providing a gradual transition in fluid velocity from the diffuser to the production tubing, and stabilizing the velocity in the case of the constant or 0.degree. exit configuration, while in the embodiments which utilize a negative exit angle, the fluid is actually being re-accelerated as it leaves the end of the diffuser. Both a 0.degree. exit angle and a negative angle serve to reduce turbulence in the fluid, and counter the tendency of a fluid to flare upon exit from the diffuser, as well as the die-swell phenomenon inherent in gels, thus reducing or eliminating the damaging erosion which is the result of these phenomena.